Sudden splutter over 50mph

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MorrisJohn
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Sudden splutter over 50mph

Post by MorrisJohn »

My 948 has twice began suddenly spluttering and jerking (quite violently) at a speed of 55mph, both times on inclines. It feels like fuel starvation. If I back off the throttle and slow the car I’m able to limp on for a few miles, spluttering and jerking on any inclines, and then things return to completely normal.

I’m able to drive the car at 50mph, uphill, downhill, whatever all day long without any problems. As evidenced by a very recent 450 mile road trip to Yorkshire.

The cars previous owner advised me he encountered the same issue. He replaced the fuel pump (modern SU type) but it didn’t fix the issue. His ‘solution’ was simply to stick to a maximum of 50mph.

Considering fuel vaporisation a possible cause I replaced the fuel hose and routed the new one around the wing, away from the manifold. No difference.

The petrol tank is only seven years old with no signs of corrosion. So it shouldn’t be flaking apart inside or full of sediment.

Has anyone else encountered this issue?
Last edited by MorrisJohn on Mon Jun 27, 2022 9:53 pm, edited 1 time in total.
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les
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Re: Sudden splutter over 50mph

Post by les »

Try another rotor arm, I had a similar thing happen, although not involving an incline.

MorrisJohn
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

les wrote: Mon Jun 27, 2022 7:11 pm Try another rotor arm, I had a similar thing happen, although not involving an incline.
Thanks Les. I’ll try that and I’ll be delighted if that’s all it is.
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kevin s
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Re: Sudden splutter over 50mph

Post by kevin s »

could also be the coil or condenser.
MorrisJohn
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

I should have mentioned, I tried changing the coil and it made no difference. I also changed the spark plugs, dizzy cap and HT leads (but not the rotor arm) as part of my spring service. All quality genuine parts. Again, no difference.

The car is on electronic ignition and aside from this one issue runs very sweetly.
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simmitc
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Re: Sudden splutter over 50mph

Post by simmitc »

Check and likely replace the air filter. A cheap and simple option. See viewtopic.php?f=4&t=49906&hilit=not+the+average+misfire
MorrisJohn
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

The air filter was also changed at spring service time - and I’ve replaced the vacuum advance pipe.
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mowogg
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Re: Sudden splutter over 50mph

Post by mowogg »

Your description would lead me to suspect its fuel.

It could be either the pump capacity or partial blockage.

I would see how quickly the r fuel pump pumps (ticks) or better still measure the delivery volume.

It could be a blockage on the suction side. What condition is the large flexible pipe in between the filler cap and tank. I have heard these can disintegrate and drop solids into the tank.
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Re: Sudden splutter over 50mph

Post by don58van »

It could be a non-vented fuel cap limiting fuel availability. That is, enough air might be seeping into the tank for all but the most demanding situations. Worth checking, I think.
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les
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Re: Sudden splutter over 50mph

Post by les »

I think we’ve covered everything!!!! :D

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Re: Sudden splutter over 50mph

Post by ManyMinors »

Also, just check that the car doesn't have an aftermarket fuel filter somewhere. These can become blocked and not allow sufficient fuel through when going up hill. There is also a small filter in the pump itself.
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

The filler hose looks fairly new and pristine externally but I’ll work my way through the list of suggestions and report back. There is no aftermarket filter fitted as I don’t like them.
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simmitc
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Re: Sudden splutter over 50mph

Post by simmitc »

If you have truly exhausted all the suggestions in this and the other thread, then I'm going to go off at a tangent (hey, don't knock it, it's worked on other topics on here).

Are you really sure that it's a misfire? Check the UJs on the propshaft. I can remember one Minor that had similar symptoms. The owner and the branch technical chap were convinced that it was a misfire, but it turned out to be the front UJ that vibrated when it reached just over 50 in 4th. There was no classic pinging sound and no obvious clunk; but it was possible to move the yoke in relation to the shaft. When the UJ was replaced, the "misfire" went away. It will cost nothing to at least look at the shaft.
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Monty-4
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Re: Sudden splutter over 50mph

Post by Monty-4 »

simmitc wrote: Tue Jun 28, 2022 12:27 pm If you have truly exhausted all the suggestions in this and the other thread, then I'm going to go off at a tangent (hey, don't knock it, it's worked on other topics on here).

Are you really sure that it's a misfire?
See also - clutch slip?
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MorrisJohn
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

If you can imagine driving along totally fine, then spluttering for a bit because you’re running out of petrol, but then somehow fresh fuel magics itself into your tank and things return to normal - that’s what it is like. I’m pretty convinced it’s fuel starvation.

I used to have a motorbike and if I ever forgot the “P” (petrol) in my “PINS” start procedure then after a few hundred yards it would splutter in the same way as it had used up what fuel was available.

Nobody has mentioned the carburettor, so I’m taking that as an unlikely source of the issue?

I’ve been a bit under the weather this week, but will hopefully get to look at it soon.
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Monty-4
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Re: Sudden splutter over 50mph

Post by Monty-4 »

It could be - I had a similar issue with a HiF38 with the cruise leaning device that would cause misfires. I think we kind of assumed you had checked the float bowl for crud, the float for leaks, the condition of the jet, the piston movement and piston damper oil?

Otherwise I would check if any thermal paste needs renewing on the electronic distributor (overheating), and perhaps trying with the old one if you can.
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Re: Sudden splutter over 50mph

Post by kevin s »

They have a gauze filter on the fuel tank pick up pipe that can clog up and is impossible to clean, blowing bacwards through the fuel line can give a temporary reprieve.
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Re: Sudden splutter over 50mph

Post by JOWETTJAVELIN »

Distributor needs checking thoroughly first. Is it even the correct one for the car. Then ensure baseplate is secure, everything clean, internal wiring including little earth lead is there and secure, points in good order and correctly gapped, carbon brush in cap present and sprung, spark plugs are correct type and not damaged.

OTHERWISE Float level in bowl. If it is an early 948 then it will have a metal ‘fork’ acting on the float and needle valve IIRC. This can be very carefully bent slightly upwards to raise the petrol level in the float.
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

Thanks very much for this. The previous owner put a Chinese made Powerspark dizzy and electronic module on it, so it’s still running on that. However, I’ve made some enquiries with sparesman. He’s got a good used original Lucas 25D that’s suitable for my 948, so I’ve told him I’ll have that and I will get the car converted back to original points.

The car is indeed an early 948 (it’s 1957). Thanks for the tip regarding the metal “fork.” I have noticed one thing curious about the float chamber. That is it does not seem to be able to tighten up fully to the carburettor. I think it’s just the one bolt that goes through to secure the float chamber to the carb? Despite it being tightened right up it (the float chamber) is still capable of moving (pivoting) a little. There are no fuel leaks from anywhere, but I know the float chamber should not move. It makes me wonder if that’s a possible factor.


JOWETTJAVELIN wrote: Tue Jun 28, 2022 9:46 pm Distributor needs checking thoroughly first. Is it even the correct one for the car. Then ensure baseplate is secure, everything clean, internal wiring including little earth lead is there and secure, points in good order and correctly gapped, carbon brush in cap present and sprung, spark plugs are correct type and not damaged.

OTHERWISE Float level in bowl. If it is an early 948 then it will have a metal ‘fork’ acting on the float and needle valve IIRC. This can be very carefully bent slightly upwards to raise the petrol level in the float.
A bad day with my Volksie still beats a good day at work!

https://www.glasgowmoggies.com
MorrisJohn
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Re: Sudden splutter over 50mph

Post by MorrisJohn »

I heard from the previous owner today. He said the car has only ever had this issue on its current fuel pump. On the previous fuel pump it ran fine at all speeds.

On the current fuel pump he had it on both points and electronic ignition at various times but it made no difference. He didn’t bother changing the pump again because he rarely reached speeds of 50mph.

On that basis I think the first thing I’ll do is swap out the fuel pump for the reconditioned club spares one, go for a run and see what happens!
A bad day with my Volksie still beats a good day at work!

https://www.glasgowmoggies.com
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